Sensor element

ABSTRACT

A sensor element includes: an inner protective layer having a porosity of 30% to 65% on two main surfaces; an intermediate protective layer, at least a part of which has contact with the inner layer, and having a porosity of 25% to 80%, which is equal to or smaller than the porosity of the inner layer; and an outer protective layer surrounding an element base on an outermost periphery on the one end portion of the sensor element, having contact with the intermediate and the inner layer, having contact with a leading end surface of the element base or the intermediate layer on the leading end surface, and having a porosity of 15% to 30%, which is smaller than the porosity of the intermediate layer, wherein a difference of porosity between the inner and the outer layer is 10% to 50%.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of PCT/JP2019/037894,filed on Sep. 26, 2019, which claims the benefit of priority of JapanesePatent Application No. 2018-187834, filed on Oct. 3, 2018, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a gas sensor detecting a predeterminedgas component in a measurement gas, and, in particular, to aconfiguration for preventing water-induced cracking of a sensor elementincluded in the gas sensor.

Description of the Background Art

As a gas sensor for determining concentration of a desired gas componentin a measurement gas, a gas sensor that includes a sensor element madeof an oxygen-ion conductive solid electrolyte, such as zirconia (ZrO₂),and including some electrodes on the surface and the inside thereof hasbeen widely known. Such a sensor element includes a protective layerformed of a porous body (porous protective layer) to prevent cracking ofthe sensor element (more particularly, an element base) occurring due tothermal shock caused by adherence of water droplets, which is so-calledwater-induced cracking. The extent of the effect of preventing thewater-induced cracking is also referred to as water resistance property.

As such a sensor element, a sensor element including protective layersprovided on opposite main surfaces of an elongated planar element base,and further including a porous protective layer provided to a leadingend portion has already been known (see Japanese Patent ApplicationLaid-Open No. 2016-48230, for example).

A gas sensor element including an elongated plate-like element having adetection part on a leading end side, a porous first protective layercovering the whole detection part, and a porous second protective layercovering an outer periphery of the first protective layer and at least arange from the leading end side of the first protective layer to a backend side than a porous layer covering an electrode located on an outerside of the element also has already been known (see Japanese Patent No.6014000, for example).

Japanese Patent Application Laid-Open No. 2016-48230 discloses thatforming the porous protective layer in a region, of the leading endportion of the sensor element, in a temperature state of 500° C. or morewhen the gas sensor is in use while not forming the porous protectivelayer in a region in a temperature state of 300° C. or less when the gassensor is in use can reduce power consumption and a waiting time untildetection due to reduction in area of formation of the porous protectivelayer, and can achieve suppression of cracking due to improvement inwater resistance property.

The sensor element according to Japanese Patent Application Laid-OpenNo. 2016-48230, however, does not necessarily have sufficient waterresistance property, and is subject to water-induced cracking in a casewhere the amount of water exposure is large.

In the gas sensor element disclosed in Japanese Patent No. 6014000 inwhich the second protective layer on the outer side covers the wholefirst protective layer on the inner side, a porosity of the secondprotective layer is small, thus there is a possibility that the secondprotective layer is peeled off on the back end side due to insufficientadhesion to an element body or water-induced cracking occurs in use at ahigh temperature.

SUMMARY

The present invention relates to a gas sensor detecting a predeterminedgas component in a measurement gas, and, in particular, to aconfiguration for preventing water-induced cracking of a sensor elementincluded in the gas sensor.

According to the present invention, a sensor element included in a gassensor detecting a predetermined gas component in a measurement gas,includes: an element base including: an elongated planar ceramic bodymade of an oxygen-ion conductive solid electrolyte, and having a gasinlet at the one end portion thereof; at least one internal chamberlocated inside the ceramic body, and communicating with the gas inletunder predetermined diffusion resistance; at least one electrochemicalpump cell including an outer pump electrode located at a location otherthan the at least one internal chamber in the ceramic body, an innerpump electrode located to face the at least one internal chamber, and asolid electrolyte located between the outer pump electrode and the innerpump electrode, the at least one electrochemical pump cell pumping inand out oxygen between the at least one internal chamber and an outside;a heater buried in a predetermined range on a side of the one endportion of the ceramic body; and an inner leading-end protective layermade up of a porous material having a porosity of 30% or more and 65% orless on at least two main surfaces facing each other on the one endportion; an intermediate leading-end protective layer, at least a partof which has contact with the inner leading-end protective layer, madeup of a porous material having a porosity of 25% or more and 80% orless, which is equal to or smaller than the porosity of the innerleading-end protective layer; and an outer leading-end protective layersurrounding the element base on an outermost periphery on the one endportion of the sensor element, having contact with the intermediateleading-end protective layer and the inner leading-end protective layeron sides of four side surfaces of the element base, having contact witha leading end surface of the element base or the intermediateleading-end protective layer on a side of the leading end surface, andmade up of a porous material having a porosity of 15% or more and 30% orless, which is smaller than the porosity of the intermediate leading-endprotective layer, wherein a difference of porosity between the innerleading-end protective layer and the outer leading-end protective layeris equal to or larger than 10% and equal to or smaller than 50%.

According to the present invention, a sensor element having greaterwater resistance property than that of a conventional sensor element andsuppressing delamination of a protective layer is achieved.

It is therefore an object of the present invention to provide a sensorelement including a porous protective layer on one end portion at whichan inlet for a measurement gas is provided, and having greater waterresistance property than that of a conventional sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic external perspective view of a sensor element 10according to a first embodiment.

FIG. 2 is a schematic diagram illustrating a configuration of a gassensor 100 including a sectional view taken along a longitudinaldirection of the sensor element 10.

FIG. 3 is a diagram for more particularly describing specific placementlocations of an outer leading-end protective layer 2 and an intermediateleading-end protective layer 3, and the significance thereof.

FIG. 4 is a diagram exemplifying a relationship between a temperatureprofile in a sensor element 10 and a configuration of the sensor element10 when the sensor element 10 is heated by the heater 150 in accordancewith a predetermined control condition when the sensor element 10 is inuse.

FIG. 5 is a diagram illustrating a flow of processing at a manufactureof the sensor element 10.

FIG. 6 is a sectional view taken along the longitudinal direction of thesensor element 20 according to a second embodiment.

FIG. 7 is a diagram for more particularly describing specific placementlocations of an outer leading-end protective layer 12 and anintermediate leading-end protective layer 3, and the significancethereof.

FIG. 8 is a diagram exemplifying a relationship between a temperatureprofile in a sensor element 20 and a configuration of the sensor element20 when the sensor element 20 is heated by the heater 150 in accordancewith a predetermined control condition when the sensor element 20 is inuse.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

<Overview of Sensor Element and Gas Sensor>

FIG. 1 is a schematic external perspective view of a sensor element (gassensor element) 10 according to a first embodiment of the presentinvention. FIG. 2 is a schematic diagram illustrating a configuration ofa gas sensor 100 including a sectional view taken along a longitudinaldirection of the sensor element 10. The sensor element 10 is a maincomponent of the gas sensor 100 detecting a predetermined gas componentin a measurement gas, and measuring concentration thereof. The sensorelement 10 is a so-called limiting current gas sensor element.

The gas sensor 100 mainly includes a pump cell power supply 30, a heaterpower supply 40, and a controller 50 in addition to the sensor element10.

As illustrated in FIG. 1, the sensor element 10 schematically has aconfiguration that a side of one end portion of an elongated planarelement base 1 is covered by a porous outer leading-end protective layer(first leading-end protective layer) 2 and an intermediate leading-endprotective layer (second leading-end protective layer) 3 also having theporous structure inside the outer-leading-end protective layer 2.

As illustrated in FIG. 2, the element base 1 is a structural body mainlyincluding an elongated planar ceramic body 101, includes a main surfaceprotective layer 170 on two main surfaces facing each other of theceramic body 101, and further includes an inner leading-end protectivelayer (third leading-end protective layer) 180 on at least the two mainsurfaces (on the main surface protective layer 170) on one end portion.In addition, in the sensor element 10, the outer leading-end protectivelayer 2 and the intermediate leading-end protective layer 3 describedabove are provided on four side surfaces and an outer side of aleading-end surface (outside the inner leading-end protective layers 180in a portion where the inner leading-end protective layers 180 arepresent) on the side of one end portion of the element base 1. Theseouter leading-end protective layer 2 and the intermediate leading-endprotective layer 3 and the inner leading-end protective layers 180 havein common that they protect the leading end portion of the element base1 from adherence of a poisoned substance and water exposure, but aredifferent in a forming method, a formation timing, and furthermore, aforming purpose and a function.

The four side surfaces of the sensor element 10 (or the element base 1,or the ceramic body 101) other than opposite end surfaces in thelongitudinal direction thereof are hereinafter simply referred to asside surfaces of the sensor element 10 (or the element base 1, or theceramic body 101). A leading end surface 101 e of the ceramic body 101is also referred to as a leading end surface 101 e of the element base1.

The ceramic body 101 is made of ceramic containing, as a main component,zirconia (yttrium stabilized zirconia), which is an oxygen-ionconductive solid electrolyte. Various components of the sensor element10 are provided outside and inside the ceramic body 101. The ceramicbody 101 having the configuration is dense and airtight. Theconfiguration of the sensor element 10 illustrated in FIG. 2 is just anexample, and a specific configuration of the sensor element 10 is notlimited to this configuration.

The sensor element 10 illustrated in FIG. 2 is a so-called serialthree-chamber structure type gas sensor element including a firstinternal chamber 102, a second internal chamber 103, and a thirdinternal chamber 104 inside the ceramic body 101. That is to say, in thesensor element 10, the first internal chamber 102 communicates, througha first diffusion control part 110 and a second diffusion control part120, with a gas inlet 105 opening to the outside on a side of one endportion E1 of the ceramic body 101 (to be precise, communicating withthe outside through the outer leading-end protective layer 2 and theintermediate leading-end protective layer 3), the second internalchamber 103 communicates with the first internal chamber 102 through athird diffusion control part 130, and the third internal chamber 104communicates with the second internal chamber 103 through a fourthdiffusion control part 140. A path from the gas inlet 105 to the thirdinternal chamber 104 is also referred to as a gas distribution part. Inthe sensor element 10 according to the present embodiment, thedistribution part is provided straight along the longitudinal directionof the ceramic body 101.

The first diffusion control part 110, the second diffusion control part120, the third diffusion control part 130, and the fourth diffusioncontrol part 140 are each provided as two slits vertically arranged inFIG. 2. The first diffusion control part 110, the second diffusioncontrol part 120, the third diffusion control part 130, and the fourthdiffusion control part 140 provide predetermined diffusion resistance toa measurement gas passing therethrough. A buffer space 115 having aneffect of buffering pulsation of the measurement gas is provided betweenthe first diffusion control part 110 and the second diffusion controlpart 120.

An external pump electrode 141 is provided on an outer surface of theceramic body 101, and an internal pump electrode 142 is provided in thefirst internal chamber 102. Furthermore, an auxiliary pump electrode 143is provided in the second internal chamber 103, and a measurementelectrode 145 is provided in the third internal chamber 104. Inaddition, a reference gas inlet 106 which communicates with the outsideand through which a reference gas is introduced is provided on a side ofthe other end portion E2 of the ceramic body 101, and a referenceelectrode 147 is provided in the reference gas inlet 106.

In a case where a target of measurement of the sensor element 10 is NOxin the measurement gas, for example, concentration of a NOx gas in themeasurement gas is calculated by a process as described below.

First, the measurement gas introduced into the first internal chamber102 is adjusted to have an approximately constant oxygen concentrationby a pumping action (pumping in or out of oxygen) of a main pump cellP1, and then introduced into the second internal chamber 103. The mainpump cell P1 is an electrochemical pump cell including the external pumpelectrode 141, the internal pump electrode 142, and a ceramic layer 101a that is a portion of the ceramic body 101 existing between theseelectrodes. In the second internal chamber 103, oxygen in themeasurement gas is pumped out of the element by a pumping action of anauxiliary pump cell P2 that is also an electrochemical pump cell, sothat the measurement gas is in a sufficiently low oxygen partialpressure state. The auxiliary pump cell P2 includes the external pumpelectrode 141, the auxiliary pump electrode 143, and a ceramic layer 101b that is a portion of the ceramic body 101 existing between theseelectrodes.

The external pump electrode 141, the internal pump electrode 142, andthe auxiliary pump electrode 143 are each formed as a porous cermetelectrode (e.g., a cermet electrode made of ZrO₂ and Pt that contains Auof 1%). The internal pump electrode 142 and the auxiliary pump electrode143 to be in contact with the measurement gas are each formed using amaterial having weakened or no reducing ability with respect to a NOxcomponent in the measurement gas.

NOx in the measurement gas caused by the auxiliary pump cell P2 to be inthe low oxygen partial pressure state is introduced into the thirdinternal chamber 104, and reduced or decomposed by the measurementelectrode 145 provided in the third internal chamber 104. Themeasurement electrode 145 is a porous cermet electrode also functioningas a NOx reduction catalyst that reduces NOx existing in the atmospherein the third internal chamber 104. During the reduction ordecomposition, a potential difference between the measurement electrode145 and the reference electrode 147 is maintained constant. Oxygen ionsgenerated by the above-mentioned reduction or composition are pumped outof the element by a measurement pump cell P3. The measurement pump cellP3 includes the external pump electrode 141, the measurement electrode145, and a ceramic layer 101 c that is a portion of the ceramic body 101existing between these electrodes. The measurement pump cell P3 is anelectrochemical pump cell pumping out oxygen generated by decompositionof NOx in the atmosphere around the measurement electrode 145. It isalso applicable that the external pump electrode 141 is not provided onthe outer surface of the ceramic body 101 but is provided an appropriateposition other than the internal chamber as long as pumping in the mainpump cell P1, the auxiliary pump cell P2, and the measurement pump cellP3 is preferably performed.

Pumping (pumping in or out of oxygen) of the main pump cell P1, theauxiliary pump cell P2, and the measurement pump cell P3 is achieved,under control performed by the controller 50, by the pump cell powersupply (variable power supply) 30 applying voltage necessary for pumpingbetween electrodes included in each of the pump cells. In a case of themeasurement pump cell P3, voltage is applied across the external pumpelectrode 141 and the measurement electrode 145 so that the potentialdifference between the measurement electrode 145 and the referenceelectrode 147 is maintained at a predetermined value. The pump cellpower supply 30 is typically provided for each pump cell.

The controller 50 detects a pump current Ip2 flowing between themeasurement electrode 145 and the external pump electrode 141 inaccordance with the amount of oxygen pumped out by the measurement pumpcell P3, and calculates a NOx concentration in the measurement gas basedon a linear relationship between a current value (NOx signal) of thepump current Ip2 and the concentration of decomposed NOx.

The gas sensor 100 preferably includes a plurality of electrochemicalsensor cells, which are not illustrated, detecting the potentialdifference between each pump electrode and the reference electrode 147,and each pump cell is controlled by the controller 50 based on a signaldetected by each sensor cell.

In the sensor element 10, a heater 150 is buried in the ceramic body101. The heater 150 is provided, below the gas distribution part in FIG.2, over a range from the vicinity of the one end portion E1 to at leasta location of formation of the measurement electrode 145 and thereference electrode 147. The heater 150 is provided mainly to heat thesensor element 10 to enhance oxygen-ion conductivity of the solidelectrolyte forming the ceramic body 101 when the sensor element 10 isin use. More particularly, the heater 150 is provided to be surroundedby an insulating layer 151.

The heater 150 is a resistance heating body made, for example, ofplatinum. The heater 150 generates heat by being powered from the heaterpower supply 40 under control performed by the controller 50.

The sensor element 10 according to the present embodiment is heated bythe heater 150 when being in use so that the temperature at least in arange from the first internal chamber 102 to the second internal chamber103 becomes 500° C. or more. In some cases, the sensor element 10 isheated so that the temperature of the gas distribution part as a wholefrom the gas inlet 105 to the third internal chamber 104 becomes 500° C.or more. These are to enhance the oxygen-ion conductivity of the solidelectrolyte forming each pump cell and to desirably demonstrate theability of each pump cell. In this case, the temperature in the vicinityof the first internal chamber 102, which becomes the highesttemperature, becomes approximately 700° C. to 800° C.

In the following description, from among the two main surfaces of theceramic body 101, a main surface (or an outer surface of the sensorelement 10 having the main surface) which is located on an upper side inFIG. 2 and on a side where the main pump cell P1, the auxiliary pumpcell P2, and the measurement pump cell P3 are mainly provided is alsoreferred to as a pump surface, and a main surface (or an outer surfaceof the sensor element 10 having the main surface) which is located on alower side in FIG. 2 and on a side where the heater 150 is provided isalso referred to as a heater surface. In other words, the pump surfaceis a main surface closer to the gas inlet 105, the three internalchambers, and the pump cells than to the heater 150, and the heatersurface is a main surface closer to the heater 150 than to the gas inlet105, the three internal chambers, and the pump cells.

A plurality of electrode terminals 160 are provided on the respectivemain surfaces of the ceramic body 101 on the other end portion E2 toestablish electrical connection between the sensor element 10 and theoutside. These electrode terminals 160 are electrically connected to theabove-mentioned five electrodes, opposite ends of the heater 150, and alead for detecting heater resistance, which is not illustrated, throughleads provided inside the ceramic body 101, which are not illustrated,to have a predetermined correspondence relationship. Application of avoltage from the pump cell power supply 30 to each pump cell of thesensor element 10 and heating by the heater 150 by being powered fromthe heater power supply 40 are thus performed through the electrodeterminals 160.

The sensor element 10 further includes the above-mentioned main surfaceprotective layers 170 (170 a, 170 b) on the pump surface and the heatersurface of the ceramic body 101. The main surface protective layers 170are layers made of alumina, having a thickness of approximately 5 μm to30 μm, and including pores with a porosity of approximately 20% to 40%,and are provided to prevent adherence of any foreign matter and poisonedsubstances to the main surfaces (the pump surface and the heatersurface) of the ceramic body 101 and the external pump electrode 141provided on the pump surface. The main surface protective layer 170 a onthe pump surface thus functions as a pump electrode protective layer forprotecting the external pump electrode 141.

In the present embodiment, the porosity is obtained by applying a knownimage processing method (e.g., binarization processing) to a scanningelectron microscope (SEM) image of an evaluation target.

The main surface protective layers 170 are provided over substantiallyall of the pump surface and the heater surface except that the electrodeterminals 160 are partially exposed in FIG. 2, but this is just anexample. The main surface protective layers 170 may locally be providedin the vicinity of the external pump electrode 141 on the one endportion E1 compared with the case illustrated in FIG. 2.

On the side of the one end portion E1 of the element base 1 included inthe sensor element 10, the above-mentioned inner leading-end protectivelayers 180 are provided on at least two main surfaces (the pump surfaceand the heater surface). The inner leading-end protective layers 180 areporous layers made of alumina, having a relatively large porosity of 30%to 65%, and having a thickness of 20 μm to 50 μm. The main surfaceprotective layers 170, however, are provided on the surface of theceramic body 101 at least in a range in which the inner leading-endprotective layers 180 are formed in the pump surface and the heatersurface.

The inner leading-end protective layers 180 have a role of preventingpoisoning and exposure to water of the sensor element 10 along with theouter leading-end protective layer 2, the intermediate leading-endprotective layer 3, and further the main surface protective layers 170.For example, the inner leading-end protective layers 180 have higherheat insulating properties than those of the outer leading-endprotective layer 2 and the main surface protective layers 170 as theyhave a large porosity second only to the intermediate leading-endprotective layer 3, and this contributes to improvement in waterresistance property of the sensor element 10.

The inner leading-end protective layers 180 also have a role asunderlying layers when the outer leading-end protective layer 2 and theintermediate leading-end protective layer 3 are formed with respect tothe element base 1. In that sense, it is only required that the innerleading-end protective layers 180 are formed, on the opposing mainsurfaces of the element base 1, at least in a range surrounded by theouter leading-end protective layer 2 and the intermediate leading-endprotective layer 3.

<Outer Leading-End Protective Layer and Intermediate Leading-EndProtective Layer>

In the sensor element 10, the outer leading-end protective layer 2 thatis a porous layer made of alumina having a purity of 99.0% or more isprovided around an outermost periphery in a predetermined range from theone end portion E1 of the element base 1 having a configuration asdescribed above, and the intermediate leading-end protective layer 3that is a porous layer made of the same type of alumina is providedbetween the outer leading-end protective layer 2 and the innerleading-end protective layers 180.

The intermediate leading-end protective layer 3 is provided along thefour side surfaces and the end surface on the one end portion E1 of theelement base 1. More specifically, as illustrated in FIG. 2, theintermediate leading-end protective layer 3 has contact with the innerleading-end protective layers 180 at least on the two main surfacesfacing each other of the element base 1, and has contact with theceramic body 101 at least on the leading end surface 101 e.

In the description hereinafter, a part of the intermediate leading-endprotective layer 3 along the side surface of the element base 1 isreferred to as a first part 3 a and a part thereof along the leading endsurface 101 e is referred to as a second part 3 b. In particular, a partof the first part 3 a along the pump surface is also referred to as apump surface-side part 3 a 1, and a part along the heater surface isalso referred to as a heater surface-side part 3 a 2. The first part 3 aand the second part 3 b, however, are not independent of each other, andare contiguous to each other. In other words, the intermediateleading-end protective layer 3 has a bottomed shape as a whole.

In the meanwhile, the outer leading-end protective layer 2 surrounds theintermediate leading-end protective layer 3 to have contact with thewhole outer surface of the intermediate leading-end protective layer 3,and has contact with the inner leading-end protective layers 180 at apart closer to a back end side than a range of formation of theintermediate leading-end protective layer 3 in a longitudinal directionof the element. Thus, the outer leading-end protective layer 2 also hasa bottomed shape as a whole.

In the description hereinafter, a portion of the outer leading-endprotective layer 2 having contact with the element base 1 is referred toas a base-fixed portion 201, a portion surrounding the side surface ofthe element base 1 and having contact with the first part 3 a of theintermediate leading-end protective layer 3 is referred to as a sidesurface portion 202, and a portion having contact with the second part 3b of the intermediate leading-end protective layer 3 is referred to asan end surface portion 203.

That is to say, the outer leading-end protective layer 2 has largelycontact with the intermediate leading-end protective layer 3, and isfixed to the element base 1 only in the base-fixed portion 201 having aband shape sequentially along the side surfaces of the element base 1.

The outer leading-end protective layer 2 and the intermediateleading-end protective layer 3 are the porous layers, thus gas flows inand out between the element base 1 (the ceramic body 101) and theoutside at all times. That is to say, introduction of the measurementgas into the element base 1 (ceramic body 101) through the gas inlet 105is naturally performed without any problems.

The outer leading-end protective layer 2 is provided to surround aportion of the element base 1 in which the temperature becomes high whenthe gas sensor 100 is in use to thereby obtain water resistance propertyin the portion. The outer leading-end protective layer 2 suppresses theoccurrence of cracking (water-induced cracking) of the element base 1due to thermal shock caused by local temperature reduction upon directexposure of the portion to water. The reason why the intermediateleading-end protective layer 3 is interposed between the outerleading-end protective layer 2 and the element base 1 is that, even ifthe outer leading-end protective layer 2 is exposed to water to causethe local temperature reduction, the interposed space having a largeheat capacity suitably suppresses the occurrence of the water-inducedcracking caused by the action of the thermal shock on the element base1.

The intermediate leading-end protective layer 3 is provided to have aporosity of 25% to 80%, and has a thickness of 100 μm to 700 μm. In themeanwhile, the outer leading-end protective layer 2 is provided to havea porosity of 15% to 30%, and has a thickness of 100 μm to 400 μm. Theintermediate leading-end protective layer 3 and the outer leading-endprotective layer 2 need not have the same thickness. The thickness ofthe outer leading-end protective layer 2 hereinafter refers to thethickness of the side surface portion 202 and the end surface portion203.

The side surface portion 202 and the end surface portion 203, however,may not have the same thickness. On the other hand, the thickness of thebase-fixed portion 201 may have a greater value than that of thethickness of the side surface portion 202 as long as the base-fixedportion 201 does not protrude farther from the side surface portion 202in an element thickness direction and an element width direction of thesensor element 10.

The thickness of only the inner leading-end protective layer 180 has asmaller value than those of both the outer leading-end protective layer2 and the intermediate leading-end protective layer 3.

These mean that the intermediate leading-end protective layer 3 which isthe porous layer having a relatively large porosity and thickness andthus having a large heat capacity and excellent in heat insulatingproperties is interposed between the outer leading-end protective layer2 and the element base 1. The provision of the intermediate leading-endprotective layer 3 has an effect that even if the outer leading-endprotective layer 2 is exposed to water to cause the local temperaturereduction, preferably suppressed is the occurrence of the water-inducedcracking caused by the action of the thermal shock on the element base1.

As an additional remark, the inner leading-end protective layer 180adjacent to the intermediate leading-end protective layer 3 is alsoformed to have a relatively large porosity of 30% to 65% as describedabove, although having a small thickness, and thus has a larger heatcapacity than the outer leading-end protective layer 2 and the mainsurface protective layers 170, although it is smaller than heat capacityof the intermediate leading-end protective layer 3. The presence of theinner leading-end protective layers 180 contributes to suppression ofthe water-induced cracking as with the intermediate leading-endprotective layer 3.

The intermediate leading-end protective layer 3 and the innerleading-end protective layer 180 have a relatively large porosity ofsubstantially the same degree, thus it is also considered that one layermade up of these protective layers may be apparently formed by a methodof forming one of these protective layers.

In case of forming such a layer, the thickness thereof is desired to belarge enough to exceed at least approximately 100 μm from a viewpoint ofsecuring the heat capacity. However, it is not easy to form such a thickfilm by an application method adopted in a formation of the innerleading-end protective layer 180 described below even if the applicationis performed repeatedly.

In the meanwhile, in a case of a thermal spraying method adopted informing the intermediate leading-end protective layer 3 and further theouter leading-end protective layer 2, the thick film is formedrelatively easily, however, the thermal spraying method is slightly lessefficient than the application method in some cases in terms of adhesion(adhesion to a layer having a relatively small porosity) of a thick filmlayer (a total thickness of the two layers is larger than 200 μm at aminimum) to be formed.

In the sensor element 10 according to the present embodiment, inconsideration of these points, the inner leading-end protective layer180 having the large porosity is provided to have the thickness of 20 μmto 50 μm by the application method on at least two main surface facingeach other of the element base 1 in the process of forming the elementbase 1, and the intermediate leading-end protective layer 3 having thelarge thickness of 100 μm to 700 μm is then provided on an outermostperiphery of the obtained element base 1 by the thermal spraying method,thus the adhesion of the intermediate leading-end protective layer 3 tothe element base 1 is secured while an advantage of the thermal sprayingmethod, that is, easiness in forming the thick film is achieved.

In addition, a difference of the porosity between the outer leading-endprotective layer 2 and the inner leading-end protective layer 180 isequal to or larger than 10% and equal to or smaller than 50%.Accordingly, a so-called anchoring effect preferably acts between thebase-fixed portion 201 of the outer leading-end protective layer 2 andthe inner leading-end protective layers 180. The anchoring effect has aneffect of suppressing delamination of the outer leading-end protectivelayer 2 from the element base 1 caused by a difference in coefficient ofthermal expansion between the outer leading-end protective layer 2 andthe element base 1 even though the adhesion between the outerleading-end protective layer 2 and the intermediate leading-endprotective layer 3 is sufficient when the sensor element 10 is in use.

That is to say, the sensor element 10 according to the presentembodiment has the configuration that the intermediate leading-endprotective layer 3 excellent in heat insulating properties is interposedbetween the outer leading-end protective layer 2 and the element base 1and the outer leading-end protective layer 2 is directly fixed to theelement base 1, and this configuration is effective in achieving boththe suppression of the water-induced cracking and securing of theadhesion of the outer leading-end protective layer 2.

The main surface protective layers 170 are made of alumina as with theinner leading-end protective layers 180, but have a smaller porosity anda smaller thickness than the inner leading-end protective layers 180,and thus, even if the inner leading-end protective layers 180 areomitted to provide the outer leading-end protective layer 2 directly onthe main surface protective layers 170, such an effect of mitigating thedifference in thermal expansion as is obtained with the innerleading-end protective layers 180 cannot highly be expected.

A porosity of the outer leading-end protective layer 2 of less than 15%is not preferable as a risk of clogging with poisoned substancesincreases, and responsiveness of the sensor element 10 is degraded.

On the other hand, a porosity of the outer leading-end protective layer2 more than 30% is not preferable as the strength of the outerleading-end protective layer 2 is not secured.

A porosity of the intermediate leading-end protective layer 3 of lessthan 25% is not preferable as a heat insulating effect cannot bepreferably obtained and thus, water resistance property is reduced.

A porosity of the inner leading-end protective layer 180 of more than65% is not preferable as adhesion to the ceramic body 101 cannot besufficiently obtained.

A portion in which the base-fixed portion 201 of the outer leading-endprotective layer 2 and the element base 1 (inner leading-end protectivelayers 180) are in contact with each other in area (a fixed area ratio)is preferably equal to or larger than 10% and equal to or smaller than50% of a total range in which the outer leading-end protective layer 2surrounds the element base 1. In this case, more stable fixing of theouter leading-end protective layer 2 to the element base 1 and securingof water resistance property are achieved. When the fixed area ratio islarger than 50%, a range of formation of the intermediate leading-endprotective layer 3 is reduced, thus such a configuration is notpreferable as the effect of securing the water resistance propertycaused by the intermediate leading-end protective layer 3 cannot besufficiently obtained.

A sum of a thickness of a part of the outer leading-end protective layer2 other than the base-fixed portion 201 and the thickness of theintermediate leading-end protective layer 3 is preferably larger thanthe thickness of the base-fixed portion 201 of the outer leading-endprotective layer 2. Accordingly, water resistance property can besecured more reliably, and power consumption is suppressed when theheater 150 performs heating.

FIG. 3 is a diagram for more particularly describing specific placementlocations of the outer leading-end protective layer 2 and theintermediate leading-end protective layer 3, and the significancethereof. As illustrated in FIG. 3, in the element base 1, three zones,that is, zones A, B, and C are conceptually defined in a longitudinaldirection of the element. Placement of the outer leading-end protectivelayer 2 and the intermediate leading-end protective layer 3 isdetermined based on these zones.

The zone A is a region heated by the heater 150 to a temperature of 500°C. or more when the gas sensor 100 is in use. As described above, whenthe gas sensor 100 is in use, the sensor element 10 is heated by theheater 150 so that the temperature at least in the range from the firstinternal chamber 102 to the second internal chamber 103 becomes 500° C.or more. The range thus belongs to the zone A at any time. FIG. 3illustrates a case where the zone A substantially coincides with aportion including the gas distribution part from the gas inlet 105 tothe third internal chamber 104 in the longitudinal direction of theelement base 1.

In contrast, the zone B is a region starting at an end portion of thefixed portion 201 on the side of the one end portion E1 (a positionfarthest from the one end portion E1 of the intermediate leading-endprotective layer 3), and ending at the other end portion E2 of theelement base 1. The zone B is maintained at 500° C. or less when the gassensor 100 is in use during which the sensor element 10 is heated by theheater 150. There is no intermediate leading-end protective layer 3 inthe zone B. More specifically, in the zone B, the temperature decreaseswith increasing distance from the one end portion E1 of the element base1, and a region in which the temperature becomes 500° C. is limited tothe vicinity of the boundary with the zone C or A.

The zone C is the region between the zones A and B in the longitudinaldirection of the element base 1. The zone C, however, is not necessarilyrequired, and the zones A and B may be adjacent to each other.

In the sensor element 10 of the gas sensor 100 according to the presentembodiment, since the base-fixed portion 201 in which the outerleading-end protective layer 2 is fixed to the inner leading-endprotective layers 180 is included in the zone B, the intermediateleading-end protective layer 3 (the first part 3 a and the second part 3b) is inevitably present at least around a portion of the element base 1belonging to the zone A, including the leading end portion.

In other words, a portion of the element base 1 heated to a hightemperature of 500° C. or more when the gas sensor 100 is in use is notin contact with the outer leading-end protective layer 2, and theintermediate leading-end protective layer 3 is surely provided aroundthe portion. When the gas sensor 100 is in use, the side surface portion202 and the end surface portion 203 of the outer leading-end protectivelayer 2 are also heated to a high temperature of 500° C. or more.

In practical use of the gas sensor 100 including the sensor element 10in which the outer leading-end protective layer 2 and the intermediateleading-end protective layer 3 are provided in a manner as describedabove, the sensor element 10 is heated by the heater 150 so that atemperature profile in which the temperature is 500° C. or more in thezone A while the temperature is 500° C. or less in the zone B isachieved.

In this heating situation, once water vapor included in the measurementgas adheres, as water droplets, to the side surface portion 202 or theend surface portion 203 of the outer leading-end protective layer 2belonging to the zone A, that is, the portion of the sensor element 10heated to a high temperature of 500° C. or more is exposed to water,local and abrupt temperature reduction occurs in the adherence portion(water-exposed portion). The side surface portion 202 and the endsurface portion 203 of the outer leading-end protective layer 2,however, are not in contact with the element base 1, and theintermediate leading-end protective layer 3 (the first part 3 a and thesecond part 3 b) having a large heat capacity is interposed betweenthem, and thus thermal shock caused by the temperature reduction in thewater-exposed portion does not occur in the element base 1. This meansthat the occurrence of the water-induced cracking of the sensor element10 is suitably prevented by using the configuration in which the porousouter leading-end protective layer 2 is provided in the portion in whichthe temperature becomes 500° C. or more when the gas sensor 100 is inuse, and the intermediate leading-end protective layer 3 is interposedbetween the outer leading-end protective layer 2 and the element base 1as in the gas sensor 100 according to the present embodiment.

It is confirmed in advance that, even if water droplets adhere to aportion in which the temperature is 500° C. or less, abrupt temperaturereduction hardly occurs, and thus thermal shock that can cause thewater-induced cracking hardly occurs.

FIG. 4 illustrates an example of the relationship between aconfiguration of the sensor element 10 and a temperature profile of thesensor element 10 when the sensor element 10 is heated by the heater 150in accordance with a predetermined control condition when the sensorelement 10 is in use. The temperature profile shown in FIG. 4 isobtained by measuring the surface temperature on the pump surface of thesensor element 10 along the longitudinal direction of the element, andplotting it with the location of the leading end surface 101 e on theone end portion E1 as the origin. Thermography is used to measure thesurface temperature.

In the example illustrated in FIG. 4, a range extending from the leadingend of the element (one end portion E1) by a distance L1 is the zone A,and a range separated from the leading end of the element base 1 by adistance L2 or more is the zone B.

If the control condition of the heater 150 is changed, the temperatureprofile of the sensor element 10 changes. The properties of the sensorelement 10, however, depend on the heating state, and thus the heater150 typically performs heating so that the same temperature profile isobtained at all times, based on one control condition fixedly set inadvance at the time of manufacture (typically, further, to exert theproperties of the element as much as possible). The sensor element 10 isthus heated so that the steady temperature profile is obtained.Accordingly, the portion of the element base 1 heated to a temperatureof 500° C. or more is the same at all times, and the ranges of the zonesA, B, and C may be considered to be fixed in each sensor element 10.

Thus, having only to specify the zones and provide the intermediateleading-end protective layer 3 and the outer leading-end protectivelayer 2 in accordance with the ranges of the zones at the manufacture ofthe sensor element 10, the intermediate leading-end protective layer 3comes to exist around the region (i.e., the zone A) every time heated bythe heater 150 to a temperature of 500° C. or more during use after themanufacture.

Furthermore, as for numerous sensor elements 10 manufactured under thesame condition, such as sensor elements 10 industrially produced inlarge quantities, when the sensor elements 10 are heated by the heaters150 under the same control condition, the temperature profiles obtainedfrom the sensor elements 10 are approximately the same as long as theyare manufactured properly. Thus, having only to specify the temperatureprofile for a sensor element 10 extracted as a sample, and to demarcatethe ranges of the zones A, B, and C based on the temperature profile, acondition for forming the outer leading-end protective layer 2 can bedetermined, based on the results, for all sensor elements 10manufactured under the same condition without actually specifying thetemperature profiles for all the sensor elements 10. That is to say, itis not necessary to actually obtain the temperature profiles for all thesensor elements 10, and demarcate the ranges of the zones A, B, and Cbased on the results.

In other words, it can be said that, for the sensor elements 10manufactured under the same condition as described above, a region (aregion to be coped with water-induced cracking) of the element base 1 isspecified in advance in accordance with setting of the control conditionof the heater 150, which is a region where the water-induced crackingmay occur upon receipt of thermal shock caused by adherence of waterdroplets during use, and thus any coping with the water-induced crackingis needed. In the case of FIGS. 3 and 4, the zone A corresponds to theregion. It can be said that the outer leading-end protective layer 2surrounds a predetermined range of the element base 1 on the one endportion E1 so that the intermediate leading-end protective layer 3 isinterposed between the region to be coped with water-induced crackingand the outer leading-end protective layer 2. It can also be said that,in this case, the outer leading-end protective layer 2 is fixed to theelement base 1 in a region specified in advance as a region(water-induced cracking not occurring region) where the water-inducedcracking does not occur during use. In the case of FIGS. 3 and 4, thezone B corresponds to the region.

As described above, according to the present embodiment, theintermediate leading-end protective layer having the large heat capacityis provided at least around the region to be coped with water-inducedcracking specified in advance and including the range from the firstinternal chamber to the second internal chamber of the element base ofthe sensor element included in the gas sensor, and the outer leading-endprotective layer is provided to surround the intermediate leading-endprotective layer. The sensor element having more superior waterresistance property than that of a conventional sensor element canthereby be achieved. Furthermore, the inner leading-end protectivelayers having a larger porosity than the outer leading-end protectivelayer are provided on at least two main surfaces facing each other inthe outer periphery of the element base, and the outer leading-endprotective layer is fixed to the inner leading-end protective layers inthe water-induced cracking not-occurring region specified in advance.Delamination and, further, detachment of the outer leading-endprotective layer can thereby suitably be suppressed.

<Process of Manufacturing Sensor Element>

One example of a process of manufacturing the sensor element 10 having aconfiguration and features as described above will be described next.FIG. 5 is a flowchart of processing at the manufacture of the sensorelement 10. As shown in FIG. 5, in the present embodiment, proceduresfor manufacturing the sensor element 10 are roughly as follows: theelement base 1 including the ceramic body 101 as a laminated body of aplurality of solid electrolyte layers is manufactured using a knowngreen sheet process (Step Sa), and then the outer leading-end protectivelayer 2 and the intermediate leading-end protective layer 3 are fixed tothe element base 1 (Step Sb). Accordingly, the ranges of the zones A, B,and C are supposed to be already known.

At the manufacture of the element base 1, a plurality of blank sheets(not illustrated) being green sheets containing the oxygen-ionconductive solid electrolyte, such as zirconia, as a ceramic componentand having no pattern formed thereon are prepared first (Step S1).

The blank sheets have a plurality of sheet holes used for positioning inprinting and lamination. The sheet holes are formed to the blank sheetsin advance prior to pattern formation through, for example, punching bya punching machine when the sheets are in the form of the blank sheets.Green sheets corresponding to a portion of the ceramic body 101 in whichan internal space is formed also include penetrating portionscorresponding to the internal space formed in advance through, forexample, punching as described above. The blank sheets are not requiredto have the same thickness, and may have different thicknesses inaccordance with corresponding portions of the element base 1 eventuallyformed.

After preparation of the blank sheets corresponding to the respectivelayers, pattern printing and drying are performed on the individualblank sheets (Step S2). Specifically, a pattern of various electrodes, apattern of the heater 150 and the insulating layer 151, a pattern of theelectrode terminals 160, a pattern of the main surface protective layers170, a pattern of internal wiring, which is not illustrated, and thelike are formed. Application or placement of a sublimable material forforming the first diffusion control part 110, the second diffusioncontrol part 120, the third diffusion control part 130, and the fourthdiffusion control part 140 is also performed at the time of patternprinting.

The patterns are printed by applying pastes for pattern formationprepared in accordance with the properties required for respectiveformation targets onto the blank sheets using known screen printingtechnology. A known drying means can be used for drying after printing.

After pattern printing on each of the blank sheets, printing and dryingof a bonding paste are performed to laminate and bond the green sheets(Step S3). The known screen printing technology can be used for printingof the bonding paste, and the known drying means can be used for dryingafter printing.

The green sheets to which an adhesive has been applied are then stackedin a predetermined order, and the stacked green sheets are crimped underpredetermined temperature and pressure conditions to thereby form alaminated body (Step S4). Specifically, crimping is performed bystacking and holding the green sheets as a target of lamination on apredetermined lamination jig, which is not illustrated, whilepositioning the green sheets at the sheet holes, and then heating andpressurizing the green sheets together with the lamination jig using alamination machine, such as a known hydraulic pressing machine. Thepressure, temperature, and time for heating and pressurizing depend on alamination machine to be used, and these conditions may be determinedappropriately to achieve good lamination.

After the laminated body is obtained as described above, the laminatedbody is cut out at a plurality of locations to obtain unit bodies(referred to as element bodies) eventually becoming the individualelement bases 1 (Step S5).

Formation (application and drying) of a pattern that becomes the innerleading-end protective layers 180 on the element base 1 at completion isthen performed on each of the cut out element bodies (Step S6).Formation of the pattern is performed using a paste prepared in advanceso that the inner leading-end protective layers 180 as desired areeventually formed.

Each of the element bodies on which the pattern that becomes the innerleading-end protective layers 180 has been formed is then fired at afiring temperature of approximately 1300° C. to 1500° C. (Step S7). Theelement base 1 is thereby manufactured. That is to say, the element base1 is generated by integrally firing the ceramic body 101 made of thesolid electrolyte, the electrodes, the main surface protective layers170, and the inner leading-end protective layers 180. Integral firing isperformed in this manner, so that the electrodes each have sufficientadhesion strength in the element base 1.

After the element base 1 is manufactured in the above-mentioned manner,formation of the outer leading-end protective layer 2 the intermediateleading-end protective layer 3 is then performed on the element base 1.

Firstly, slurry containing a material for forming the intermediateleading-end protective layer 3 is thermal sprayed onto the element base1 at a formation target location of the intermediate leading-endprotective layer 3, and then dried (step S11). Accordingly, an organiccomponent volatilizes from a thermal sprayed film, and the intermediateleading-end protective layer 3 is formed.

Subsequently, slurry containing a material for forming the outerleading-end protective layer 2 is thermal sprayed onto the element base1 at a formation target location of the outer leading-end protectivelayer 2, and then dried (step S12). Accordingly, an organic componentvolatilizes from a thermal sprayed film, and the outer leading-endprotective layer 2 is formed.

Slurry used in each thermal spraying is made of alumina powder, binder,and solvent, for example, and is prepared in advance in accordance withthe porosity achieved in each layer.

The sensor element 10 is thereby obtained. The sensor element 10 thusobtained is housed in a predetermined housing, and built into the body,which is not illustrated, of the gas sensor 100.

Second Embodiment

The configuration of the sensor element for securing water resistanceproperty by interposing the intermediate leading-end protective layerbetween the outer leading-end protective layer and the element basewhile suppressing delamination and detachment of the outer leading-endprotective layer is not limited to that shown in the first embodiment.In the present embodiment, a configuration of a sensor element 20 heatedin accordance with a temperature profile shifted to a lower temperatureside compared with the sensor element 10 according to the firstembodiment is described.

FIG. 6 is a sectional view taken along the longitudinal direction of thesensor element 20 according to a second embodiment of the presentinvention. The sensor element 20 has components similar to those of thesensor element 10 according to the first embodiment except for somecomponents. The similar components thus bear the same reference signs asthose in the first embodiment, and detailed description thereof isomitted below.

As with the sensor element 10 according to the first embodiment, thesensor element 20 is used as a main component of the gas sensor 100,under operation control of the pump cells and the heater 150 performedthrough control of the pump cell power supply 30 and the heater powersupply 40 performed by the controller 50. Thus, in a case where thetarget of measurement of the gas sensor 100 is NOx in the measurementgas, operation of the pump cells and the heater 150 of the sensorelement 20 is controlled through control of the pump cell power supply30 and the heater power supply 40 performed by the controller 50, andthe NOx concentration in the measurement gas is calculated by thecontroller 50 based on the linear relationship between the current value(NOx signal) of the pump current 1 p 2 flowing through the measurementpump cell P3 under the control and the concentration of decomposed NOx.

As illustrated in FIG. 6, the sensor element 20 includes, in place ofthe outer leading-end protective layer 2 of the sensor element 10, anouter leading-end protective layer (first leading-end protective layer)12 fixed to the element base 1 in a different manner from the outerleading-end protective layer 2. Specifically, the outer leading-endprotective layer 12 of the sensor element 20 is similar to the outerleading-end protective layer 2 of the sensor element 10 in that theintermediate leading-end protective layer 3 is interposed between theouter leading-end protective layer and the side surfaces of the elementbase 1. The outer leading-end protective layer 12, however, is differentfrom the outer leading-end protective layer 2 in that an end surfaceportion 204 is fixed to the leading end surface 101 e of the elementbase 1 on the one end portion E1 of the element base 1, as the endsurface portion 203 of the outer leading-end protective layer 2 isseparated from the element base. The intermediate leading-end protectivelayer 3 existing in the sensor element 20 is thus only the first part 3a interposed between the outer leading-end protective layer 12 and theside surfaces of the element base 1, and the second part 3 b interposedin the sensor element 10 is not present. Since the outer leading-endprotective layer 12 and the intermediate leading-end protective layer 3are porous layers, introduction of the measurement gas into the elementbase 1 (ceramic body 101) through the gas inlet 105 is performed withoutany problems.

That is to say, the outer leading-end protective layer 12 included inthe sensor element 20 according to the present embodiment has contactwith the intermediate leading-end protective layer 3 in the side surfaceportion 202, and is fixed to the element base 1 in the base-fixedportion 201 having a band shape sequentially along the side surfaces ofthe element base 1 and the end surface portion 204.

Also in the outer leading-end protective layer 12, a portion in whichthe base-fixed portion 201 and the element base 1 (inner leading-endprotective layers 180) are in contact with each other in area (a fixedarea ratio) is preferably equal to or larger than 10% and smaller than50% of a total range in which the outer leading-end protective layer 12surrounds the element base 1, as with the outer leading-end protectivelayer 2 of the sensor element 10.

The sensor element 20 having a configuration as described above can bemanufactured in a similar manner to the sensor element 10 according tothe first embodiment as described based on FIG. 5 except that the mannerof formation of the thermal sprayed films of them is different due tothe difference in shape of the intermediate leading-end protective layer3 and the outer leading-end protective layer 12 eventually formed.

The difference between the sensor element 10 and the sensor element 20in presence or absence of the interposed second part 3 b corresponds tothe difference between them in temperature profile when the gas sensor100 is in use. As described above, the sensor element 20 according tothe present embodiment is assumed to be used in accordance with thetemperature profile shifted to the lower temperature side compared withthe sensor element 10 according to the first embodiment. Description ismade on this point based on FIG. 7. FIG. 7 is a diagram for moreparticularly describing specific placement locations of the outerleading-end protective layer 12 and the intermediate leading-endprotective layer 3, and the significance thereof, similarly to FIG. 3.

Placement of the outer leading-end protective layer 12 and theintermediate leading-end protective layer 3 is determined based on zonesdividing the element base 1 also in the case of the sensor element 20 aswith the sensor element 10. As illustrated in FIG. 7, the sensor element20 has the zones A, B, and C as with the sensor element 10. Definitionsof these zones are the same as those in the sensor element 10. That isto say, the zone A is the region at least including the range from thefirst internal chamber 102 to the second internal chamber 103, andheated by the heater 150 to a temperature of 500° C. or more when thegas sensor 100 is in use. The zone B is the region starting at the endportion of the base-fixed portion 201 in which the outer leading-endprotective layer 12 is fixed to the inner leading-end protective layers180 on the side of the one end portion E1, and ending at the other endportion E2 of the element base 1, and maintained at 500° C. or less whenthe gas sensor 100 is in use. The zone C is the region between the zonesA and B in the longitudinal direction of the element base 1.

While the zone A reaches the gas inlet 105 in the sensor element 10illustrated in FIG. 3, a predetermined range extending from the gasinlet 105 is classified as a zone D different from the zone A in thesensor element 20 illustrated in FIG. 7.

The zone D is a region maintained at 500° C. or less when the gas sensor100 is in use on the one end portion E1 of the sensor element 20. Inother words, when the gas sensor 100 including the sensor element 20 isin use, the sensor element 20 is heated by the heater 150 providedinside the sensor element 20 so that the temperature profile in whichthe zone D is formed in addition to the zones A to C is achieved.

In the sensor element 20, the intermediate leading-end protective layer3 (first part 3 a) is inevitably present at least around the portion ofthe element base 1 belonging to the zone A as in the sensor element 10.Thus, once the portion belonging to the zone A and heated to a hightemperature of 500° C. or more is exposed to water when the gas sensor100 is in use, local and abrupt temperature reduction occurs in thewater-exposed portion, but thermal shock caused by the temperaturereduction in the water-exposed portion does not occur in the elementbase 1. This is because the side surface portion 202 of the outerleading-end protective layer 12 is not in contact with the element base1 and the intermediate leading-end protective layer 3 (first part 3 a)having a large heat capacity is interposed between them.

This is also as in the sensor element 10 that, even if water dropletsadhere to the portion in which the temperature is 500° C. or less whenthe gas sensor 100 is in use, abrupt temperature reduction hardlyoccurs, and thus thermal shock that may cause the water-induced crackinghardly occurs. In the sensor element 20, such a portion in which thetemperature is 500° C. or less during use is present not only in thezone B on the side of the other end portion E2, but also in the zone Don the side of the one end portion E1.

A suitable range of the thickness and the porosity of the outerleading-end protective layer 12 is similar to that of the outerleading-end protective layer 2 of the sensor element 10. A range of thethickness and the porosity of the intermediate leading-end protectivelayer 3 is also similar to that of the sensor element 10.

FIG. 8 illustrates an example of the relationship between aconfiguration of the sensor element 20 and a temperature profile of thesensor element 20 when the sensor element 20 is heated by the heater 150in accordance with a predetermined control condition when the sensorelement 20 is in use. The temperature profile shown in FIG. 8 isobtained by measuring the surface temperature on the pump surface of thesensor element 20 along the longitudinal direction of the element, andplotting it with the location of the leading end surface 101 e on theone end portion E1 as the origin. Thermography is used to measure thesurface temperature.

In the example illustrated in FIG. 8, a range extending from the leadingend surface 101 e by a distance L3 is the zone D, and a range adjacentto the range and extending from the location of the distance L3 to thelocation of the distance L1 is the zone A, in contrast to the case ofFIG. 4. A range separated from the leading end of the element by thedistance L2 or more is the zone B.

Having only to specify the zones and provide the intermediateleading-end protective layer 3 and the outer leading-end protectivelayer 12 in accordance with the ranges of the zones at the manufactureof the sensor element 20, the intermediate leading-end protective layer3 comes to exist around the region (i.e., the zone A) every time heatedby the heater 150 to a temperature of 500° C. or more during use afterthe manufacture.

As in the sensor element 10, as for numerous sensor elements 20manufactured under the same condition, such as sensor elements 20industrially produced in large quantities, having only to specify thetemperature profile for a sensor element 20 extracted as a sample, andto demarcate the ranges of the zones A, B, C, and D based on thetemperature profile, a condition for forming the outer leading-endprotective layer 12 and the intermediate leading-end protective layer 3can be determined, based on the results, for all sensor elements 20manufactured under the same condition without actually specifying thetemperature profiles for all the sensor elements 20. That is to say, itis not necessary to actually obtain the temperature profiles for all thesensor elements 20, and demarcate the ranges of the zones A, B, C, and Dbased on the results.

In other words, it can be said that, for the sensor elements 20manufactured under the same condition as described above, the region tobe coped with water-induced cracking of the element base 1 is specifiedin advance in accordance with setting of the control condition of theheater 150, as in the sensor element 10. In the case of FIGS. 7 and 8,the zone A corresponds to the region. The sensor element 20, however,differs from the sensor element 10 in that such a region exists only ina part of the side surfaces of the element base 1. It can be said thatthe outer leading-end protective layer 12 surrounds a predeterminedrange of the element base 1 on the one end portion E1 so that theintermediate leading-end protective layer 3 is interposed between theregion to be coped with water-induced cracking and the outer leading-endprotective layer 12. In this case, the outer leading-end protectivelayer 12 is fixed to the element base 1 in the water-induced crackingnot-occurring region on the side surfaces of the element base 1 as inthe sensor element 10. In the case of FIGS. 7 and 8, the zone Bcorresponds to the region. The sensor element 20, however, differs fromthe sensor element 10 in that the outer leading-end protective layer 12is further fixed to the leading end surface 101 e of the element base 1.

Also in a case where the temperature on the one end portion E1 becomes500° C. or less as in the sensor element 20 of FIG. 8, the intermediateleading-end protective layer 3 and the outer leading-end protectivelayer 2 may be provided so that the second part 3 b is interposedbetween the outer leading-end protective layer 2 and the element base 1as in the sensor element 10 according to the first embodiment. This isbecause the intermediate leading-end protective layer 3 is still presentaround the zone A.

As described above, also according to the present embodiment, in themanner similar to the first embodiment, the intermediate leading-endprotective layer having the large heat capacity is provided at leastaround the region to be coped with water-induced cracking specified inadvance and including the range from the first internal chamber to thesecond internal chamber of the element base of the sensor elementincluded in the gas sensor, and the outer leading-end protective layeris provided to surround the intermediate leading-end protective layer.The sensor element excellent in water resistance property is therebyachieved.

Modification Example

The above-mentioned embodiments are targeted at a sensor element havingthree internal chambers, but the sensor element may not necessarily havea three-chamber configuration. That is to say, the configuration inwhich the inner leading-end protective layers having a large porosityare provided on outermost surfaces of the element base on the endportion at least including the gas distribution part, and, further, theouter leading-end protective layer as the porous layer having a smallerporosity than the inner leading-end protective layers is providedoutside the inner leading-end protective layers so that the intermediateleading-end protective layer is interposed between the outer leading-endprotective layer and the portion of the element base in which thetemperature becomes 500° C. or more during use is applicable to a sensorelement having one internal chamber or two internal chambers.

In the above-mentioned embodiments, the region heated to a temperatureof 500° C. or more during use is set to the region to be coped withwater-induced cracking on the premise of the configuration of the sensorelement illustrated in FIG. 2 or 6, but the heating temperature of theregion considerable as a target of the region to be coped withwater-induced cracking may vary depending on the configuration of thesensor element.

Examples

(Test 1)

As the sensor element 10 according to the first embodiment, eight typesof sensor elements 10 (Examples 1 to 8) having different combinations ofthicknesses (thicknesses of the side surface portion 202 and the endsurface portion 203) and porosity of the outer leading-end protectivelayer 2 and thicknesses (thickness of the first part 3 a and second part3 b) and porosity of the intermediate leading-end protective layer 3were manufactured, and a test of water resistance was conducted on them.

As comparative examples, a sensor element (Comparative example 1) inwhich the outer leading-end protective layer 2 as a whole adhered to theelement base 1 without the intermediate leading-end protective layer 3being interposed therebetween and a sensor element (Comparative example2) in which the outer leading-end protective layer 2 and theintermediate leading-end protective layer 3 were not provided to exposethe element base 1 were manufactured, and a similar test was conductedon them.

Table 1 lists the thickness of the outer leading-end protective layer 2,the thickness of the intermediate leading-end protective layer 3, theporosity of the outer leading-end protective layer 2, the porosity ofthe intermediate leading-end protective layer 3, and the results ofdetermination in the water resistance test for each sensor element. Theelement bases 1 of all the sensor elements were manufactured under thesame condition. The sensor elements according to Examples 1 to 8 wereeach set to have a fixed area ratio of 30%. The inner leading-endprotective layers 180 of all the sensor elements have the porosity of40% and the thickness of 40 μm.

TABLE 1 Thickness of Porosity Porosity of Thickness of intermediate ofouter intermediate outer leading-end leading-end leading-end leading-endDeter- protective layer protective layer protective layer protectivelayer mination Level [μm] [μm] [%] [%] 1 Example 1 200 200 20 30 ◯Example 2 300 200 15 40 ⊚ Example 3 150 150 25 40 ⊚ Example 4 200 200 1525 ◯ Example 5 150 200 30 50 ⊚ Example 6 200 500 25 65 ⋆ Example 7 200350 30 55 ⋆ Example 8 200 700 25 80 ⋆ Comparative 300 — 20 — × example 1Comparative — — — — × example 2

The water resistance test was conducted by the following procedures.First, the heater 150 was energized to heat the sensor element 10 sothat a temperature profile in which a maximum temperature in the zone Awas 800° C., and the temperature in the zone B was 500° C. or less wasobtained. In the temperature profile, the range from the gas inlet 105to the third internal chamber 104 in the longitudinal direction of theelement belonged to the zone A.

While the heating state was maintained, the pump cells and, further, thesensor cells of the sensor element were operated in ambient atmosphereto perform control so that oxygen concentration in the first internalchamber 102 was maintained at a predetermined constant value to therebyobtain a situation in which a pump current Ip0 in the main pump cell P1was stabilized.

Under the situation, a predetermined amount of water was dropped ontothe side surface portion 202 of the outer leading-end protective layer 2belonging to the zone A (onto a corresponding portion of the elementbase 1 in Comparative example 2), and whether a change of the pumpcurrent Ip0 before and after dropping exceeded a predetermined thresholdwas determined. If the change of the pump current Ip0 did not exceed thethreshold, the amount of dropped water was increased to repeat thedetermination. The amount of dropped water when the change of the pumpcurrent Ip0 eventually exceeded the threshold was defined as a crackingoccurring dropped water amount, and water resistance property or a lackthereof was determined based on the magnitude of a value of the crackingoccurring dropped water amount. Determination in this manner wasreferred to as Determination 1. A maximum value of the amount of droppedwater was set to 40 μL.

In this test, the change of the pump current Ip0 was used as a criterionfor determining the occurrence of cracking in the element base 1. Thisutilizes such a causal relationship that, when cracking of the elementbase 1 occurs due to thermal shock caused by dropping (adherence) ofwater droplets onto the outer leading-end protective layer 2, oxygenflows into the first internal chamber 102 through a portion of thecracking, and the value of the pump current Ip0 increases.

Specifically, the sensor element was determined to have excellent waterresistance property if the cracking occurring dropped water amount was20 μL or more. The sensor element was determined to have great waterresistance property if the cracking occurring dropped water amount was15 μL or more and less than 20 μL. The sensor element was determined tohave water resistance property in a range allowable in practical use ifthe cracking occurring dropped water amount was 10 μL or more and lessthan 15 μL. The sensor element was determined to have insufficient waterresistance property in terms of practicality if the cracking occurringdropped water amount was less than 10 μL. In Japanese Patent ApplicationLaid-Open No. 2016-48230, a case where cracking does not occur with anamount of dropped water of 3 μL is determined as an example. If thecracking occurring dropped water amount is 10 μL or more, the sensorelement is thus determined to have more superior water resistanceproperty than that of a conventional sensor element.

In the sensor element including the outer leading-end protective layer2, delamination of the outer leading-end protective layer 2 in thebase-fixed portion 201 did not occur until cracking of the element base1 occurred.

In Table 1, as the results of Determination 1, a star is marked for thesensor element in which the cracking occurring dropped water amount is20 μL or more or cracking does not occur upon dropping of a maximumamount of water, a double circle is marked for the sensor element inwhich the cracking occurring dropped water amount is 15 μL or more andless than 20 μL, a single circle is marked for the sensor element inwhich the cracking occurring dropped water amount is 10 μL or more andless than 15 μL, and a cross is marked for the sensor element in whichthe cracking occurring dropped water amount is less than 10 μL.

According to the results shown in Table 1, the sensor elements inExamples 1 to 5 are each marked with the double circle or the singlecircle, and the sensor elements in Examples 6 to 8 are each marked withthe star, whereas the sensor elements in Comparative examples 1 and 2are each marked with the cross. The cracking occurring dropped wateramounts in Examples 6 to 8 were 30 μL, 20 μL, and 40 μL, respectively.In the meanwhile, it was determined that cracking occurred in the sensorelement in Comparative example 1 with an amount of dropped water of 5 μLto 9 μL. It was also determined that cracking occurred in the sensorelement in Comparative example 2 with an amount of dropped water of lessthan 1 μL.

The results shown in Table 1 indicate that the sensor element havingmore superior water resistance property than the conventional sensorelement can be achieved by providing the intermediate leading-endprotective layer as the porous layer having a porosity ranging from 25%to 80% and a thickness of 100 μm or more and 700 μm or less at leastaround the portion of the element base of the sensor element included inthe gas sensor heated to a high temperature of 500° C. or more when thegas sensor is in use and the outer leading-end protective layer as theporous layer having a porosity ranging from 15% to 30% and a thicknessof 100 μm or more and 400 μm or less outside the intermediateleading-end protective layer as in the first embodiment, for example.

(Test 2)

A test was conducted to determine the influence of difference ofporosity between the outer leading-end protective layer 2 and the innerleading-end protective layer 180 on water resistance property andadhesion between the outer leading-end protective layer 2 and theelement base 1 and between the intermediate leading-end protective layer3 and the element base 1. Specifically, the porosity of the outerleading-end protective layer 2 was set in a range of 15% to 30%, theporosity of the intermediate leading-end protective layer 3 was set in arange of 25% to 80%, and the porosity of the inner leading-endprotective layer 180 was set in a range of 30% to 65%. Eight types ofsensor elements (Examples 9 to 16) having different combinations ofvalues thereof were manufactured, and a test of water resistanceproperty and adhesion of the protective layer was conducted on them. Inany sensor element, the porosity of the outer leading-end protectivelayer 2 and the porosity of the inner leading-end protective layer 180were set within a range of 10% to 50%.

Used as the sensor elements of Examples 9 to 13 were sensor elementsmanufactured in the same conditions (the same porosity and thickness) asthose of the sensor elements of Examples 1 to 5, respectively. In thesensor element of Example 14, the outer leading-end protective layer 2,the intermediate leading-end protective layer 3, and the innerleading-end protective layer 180 have the thickness of 200 μm, 200 μm,and 50 μm, respectively. In the sensor element of Examples 15 and 16,the outer leading-end protective layer 2, the intermediate leading-endprotective layer 3, and the inner leading-end protective layer 180 havethe thickness of 200 μm, 700 μm, and 50 μm, respectively.

As comparative examples, a sensor element (Comparative example 3) inwhich the inner leading-end protective layer 180 was not provided andthe two types of sensor elements (Comparative examples 4 and 5) in whichtwo of the outer leading-end protective layer 2, the intermediateleading-end protective layer 3, and the inner leading-end protectivelayer 180 had the porosity out of the setting range described above weremanufactured, and a similar test was conducted on them.

The water resistance test was conducted by procedures similar to thosein Test 1 except that conduction state by the heater 150 and locationsof water droplets were different.

The conduction by the heater 150 was performed so that a surfacetemperature (maximum temperature) in a portion in which the intermediateleading-end protective layer 3 was interposed between the outerleading-end protective layer 2 and the inner leading-end protectivelayer 180 (intermediate leading-end protective layer interposedportion), which belongs to the zone A, was variously different in arange of 700° C. to 800° C., and a surface temperature in a portion inwhich the intermediate leading-end protective layer 3 was not interposedbetween the outer leading-end protective layer 2 and the innerleading-end protective layer 180 (intermediate leading-end protectivelayer non-interposed portion), which belongs to the zone B, wasvariously different in a range of 350° C. to 500° C. The surfacetemperature was measured by thermography.

A sensor element made under the same condition as that of Example 9(Example 1) except that only the conduction state by the heater 150 wasdifferent and the surface temperature in the intermediate leading-endprotective layer non-interposed portion was 600° C. was also made as acomparative example (Comparative example 6).

Water was dropped at two locations, that is, the surface of the sidesurface portion 202 corresponding to the intermediate leading-endprotective layer interposed portion as with Test 1, and the surface ofthe base-fixed portion 201 corresponding to the intermediate leading-endprotective layer non-interposed portion. Determinations of waterresistance property or a lack thereof in each location were referred asDetermination 1 and Determination 2.

Evaluation of adhesion was performed by performing a heating vibrationtest, and subsequently determining presence or absence of delaminationof the outer leading-end protective layer 2 and the intermediateleading-end protective layer 3 by visual observation.

The heating vibration test was performed under the following conditionsin a state where each sensor element was attached to an exhaust pipe ofa propane burner disposed in a vibration-testing machine.

Gas temperature: 850° C.;

Gas air ratio λ: 1.05;

Vibration condition: sweeping with 50 Hz→100 Hz→150 Hz→250 Hz for thirtyminutes;

Acceleration rate: 30 G, 40 G, 50 G;

Test time: 150 hours.

Table 2 shows, for each sensor element, the porosity of each of theouter leading-end protective layer 2, the intermediate leading-endprotective layer 3, and the inner leading-end protective layer 180, thedifference of porosity between the outer leading-end protective layer 2and the inner leading-end protective layer 180, the surface temperatureof each of the intermediate leading-end protective layer interposedportion and the intermediate leading-end protective layer non-interposedportion, a result of water resistance test determined by the samecriterion as that of Test 1 (Determination 1 and Determination 2), and adetermination result of adhesion (Determination 3). According to thedetermination result of the adhesion, the sensor elements in which thedelamination was not confirmed are each marked with the single circle,whereas the sensor elements in which the delamination was confirmed areeach marked with the cross.

TABLE 2 Difference of Surface Surface Porosity of Porosity of Porosityof porosity temperature temperature of outer intermediate inner betweenouter of intermediate intermediate leading-end leading-end leading-endleading-end protective leading-end leading-end Deter- Deter- Deter-protective protective protective layer and inner protective layerprotective layer mina- mina- mina- layer layer layer leading-endprotective interposed portion non-interposed tion tion tion Level [%][%] [%] layer [%] [° C.] portion [° C.] 1 2 3 Example 9  20 30 35 15 850450 ◯ ◯ ◯ Example 10 15 40 50 35 700 350 ⊚ ⊚ ◯ Example 11 25 40 45 20750 450 ⊚ ◯ ◯ Example 12 15 25 30 15 700 350 ◯ ⊚ ◯ Example 13 30 50 6030 850 500 ⊚ ⊚ ◯ Example 14 15 40 65 50 830 450 ⊚ ⊚ ◯ Example 15 25 8040 15 800 350 ⋆ ⊚ ◯ Example 16 20 55 50 30 850 470 ⋆ ⊚ ◯ Comparative 2540 None — 750 450 ⊚ x x example 3 Comparative 15 20 20  5 850 450 ◯ ◯ xexample 4 Comparative 10 30 70 60 830 450 ◯ ⊚ x example 5 Comparative 2030 35 15 900 600 ◯ x ◯ example 6

In Table 2, the sensor elements in Examples 9 to 14 are each marked withthe star, the double circle, or the single circle in all Determinations1 to 3. In these sensor elements, the difference of porosity between theouter leading-end protective layer 2 and the inner leading-endprotective layer 180 is within a range of 10% to 50%.

In contrast, the sensor elements in Comparative examples 3 to 6 are eachmarked with the double circle or the single circle in Determination 1,but marked with the cross in at least one of Determinations 2 and 3.

More specifically, shown is a result that the sensor elements inComparative example 3 in which the inner leading-end protective layer180 is not provided and Comparative example 6 in which the surfacetemperature in the intermediate leading-end protective layernon-interposed portion is 600° C. are each marked with the cross inDetermination 2. In comparison between the result and Examples 9 to 16,it is considered necessary to provide the inner leading-end protectivelayer 180 and further make the base-fixed portion 201 in which the outerleading-end protective layer 2 is directly fixed to the innerleading-end protective layer 180 belong to the zone B maintained at 500°C. or less when the sensor element is in use from a viewpoint of theeffect of securing the water resistance property.

Shown is a result that the sensor elements in Comparative example 3 inwhich the inner leading-end protective layer 180 is not provided andComparative examples 4 and 5 in which the difference of porosity betweenthe outer leading-end protective layer 2 and the inner leading-endprotective layer 180 are 5% and 60%, respectively, are each marked withthe cross in Determination 3. Specifically, in these sensor elements,delamination occurs at least between the base-fixed portion 201 of theouter leading-end protective layer 2 and the inner leading-endprotective layer 180.

Also in consideration of Examples 9 to 16, it is considered that theresults indicate that when the difference of porosity between the outerleading-end protective layer 2 and the inner leading-end protectivelayer 180 is within a range of 10% to 50%, anchoring effect preferablyacts between the base-fixed portion 201 and the inner leading-endprotective layers 180, thus the adhesion of the outer leading-endprotective layer 2 and the intermediate leading-end protective layer 3to the element base 1 is secured.

It is considered that a value in a range of 25% to 80% suffices as theporosity of the intermediate leading-end protective layer 3 at thattime.

What is claimed is:
 1. A sensor element included in a gas sensordetecting a predetermined gas component in a measurement gas,comprising: an element base including: an elongated planar ceramic bodymade of an oxygen-ion conductive solid electrolyte, and having a gasinlet at the one end portion thereof; at least one internal chamberlocated inside the ceramic body, and communicating with the gas inletunder predetermined diffusion resistance; at least one electrochemicalpump cell including an outer pump electrode located at a location otherthan the at least one internal chamber in the ceramic body, an innerpump electrode located to face the at least one internal chamber, and asolid electrolyte located between the outer pump electrode and the innerpump electrode, the at least one electrochemical pump cell pumping inand out oxygen between the at least one internal chamber and an outside;a heater buried in a predetermined range on a side of the one endportion of the ceramic body; and an inner leading-end protective layermade up of a porous material having a porosity of 30% or more and 65% orless on at least two main surfaces facing each other on the one endportion; an intermediate leading-end protective layer, at least a partof which has contact with the inner leading-end protective layer, madeup of a porous material having a porosity of 25% or more and 80% orless, which is equal to or smaller than the porosity of the innerleading-end protective layer; and an outer leading-end protective layersurrounding the element base on an outermost periphery on the one endportion of the sensor element, having contact with the intermediateleading-end protective layer and the inner leading-end protective layeron sides of four side surfaces of the element base, having contact witha leading end surface of the element base or the intermediateleading-end protective layer on a side of the leading end surface, andmade up of a porous material having a porosity of 15% or more and 30% orless, which is smaller than the porosity of the intermediate leading-endprotective layer, wherein a difference of porosity between the innerleading-end protective layer and the outer leading-end protective layeris equal to or larger than 10% and equal to or smaller than 50%.
 2. Thesensor element according to claim 1, wherein the intermediateleading-end protective layer is provided to have contact with a regionto be coped with water-induced cracking, which is specified in advance,in the element base, and the outer leading-end protective layer hascontact with the inner leading-end protective layer in a water-inducedcracking not-occurring region, which is specified in advance, in theelement base.
 3. The sensor element according to claim 2, wherein theregion to be coped with water-induced cracking is a region in theelement base heated to a temperature of 500° C. or more when the gassensor is in use, and a portion in which the outer leading-endprotective layer and the inner leading-end protective layer have contactwith each other is disposed in a portion maintained at 500° C. or lesswhen the gas sensor is in use.
 4. The sensor element according to claim2, wherein the intermediate leading-end protective layer has contactwith a part of an outer surface of the inner leading-end protectivelayer and a leading-end surface of the element base, and the outerleading-end protective layer has contact with the intermediateleading-end protective layer also in the side of the leading-end surfaceof the element base.
 5. The sensor element according to claim 2, whereinthe intermediate leading-end protective layer has contact with a part ofan outer surface of the inner leading-end protective layer, and theouter leading-end protective layer has contact with the leading-endsurface of the element base in the side of the leading-end surface. 6.The sensor element according to claim 1, wherein a thickness of theinner leading-end protective layer is equal to or larger than 20 μm andequal to or smaller than 50 μm, a thickness of the intermediateleading-end protective layer is equal to or larger than 100 μm and equalto or smaller than 700 μm, and a thickness of the outer leading-endprotective layer is equal to or larger than 100 μm and equal to orsmaller than 400 μm.
 7. The sensor element according to claim 1, whereina portion in which the outer leading-end protective layer and the innerleading-end protective layer are in contact with each other in area isequal to or larger than 10% and equal to or smaller than 50% of a rangein which the outer leading-end protective layer surrounds the elementbase in area.
 8. The sensor element according to claim 3, wherein theintermediate leading-end protective layer has contact with a part of anouter surface of the inner leading-end protective layer and aleading-end surface of the element base, and the outer leading-endprotective layer has contact with the intermediate leading-endprotective layer also in the side of the leading-end surface of theelement base.
 9. The sensor element according to claim 3, wherein theintermediate leading-end protective layer has contact with a part of anouter surface of the inner leading-end protective layer, and the outerleading-end protective layer has contact with the leading-end surface ofthe element base in the side of the leading-end surface.
 10. The sensorelement according to claim 2, wherein a thickness of the innerleading-end protective layer is equal to or larger than 20 μm and equalto or smaller than 50 μm, a thickness of the intermediate leading-endprotective layer is equal to or larger than 100 μm and equal to orsmaller than 700 μm, and a thickness of the outer leading-end protectivelayer is equal to or larger than 100 μm and equal to or smaller than 400μm.
 11. The sensor element according to claim 3, wherein a thickness ofthe inner leading-end protective layer is equal to or larger than 20 μmand equal to or smaller than 50 μm, a thickness of the intermediateleading-end protective layer is equal to or larger than 100 μm and equalto or smaller than 700 μm, and a thickness of the outer leading-endprotective layer is equal to or larger than 100 μm and equal to orsmaller than 400 μm.
 12. The sensor element according to claim 4,wherein a thickness of the inner leading-end protective layer is equalto or larger than 20 μm and equal to or smaller than 50 μm, a thicknessof the intermediate leading-end protective layer is equal to or largerthan 100 μm and equal to or smaller than 700 μm, and a thickness of theouter leading-end protective layer is equal to or larger than 100 μm andequal to or smaller than 400 μm.
 13. The sensor element according toclaim 5, wherein a thickness of the inner leading-end protective layeris equal to or larger than 20 μm and equal to or smaller than 50 μm, athickness of the intermediate leading-end protective layer is equal toor larger than 100 μm and equal to or smaller than 700 μm, and athickness of the outer leading-end protective layer is equal to orlarger than 100 μm and equal to or smaller than 400 μm.
 14. The sensorelement according to claim 2, wherein a portion in which the outerleading-end protective layer and the inner leading-end protective layerare in contact with each other in area is equal to or larger than 10%and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.
 15. Thesensor element according to claim 3, wherein a portion in which theouter leading-end protective layer and the inner leading-end protectivelayer are in contact with each other in area is equal to or larger than10% and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.
 16. Thesensor element according to claim 4, wherein a portion in which theouter leading-end protective layer and the inner leading-end protectivelayer are in contact with each other in area is equal to or larger than10% and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.
 17. Thesensor element according to claim 5, wherein a portion in which theouter leading-end protective layer and the inner leading-end protectivelayer are in contact with each other in area is equal to or larger than10% and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.
 18. Thesensor element according to claim 6, wherein a portion in which theouter leading-end protective layer and the inner leading-end protectivelayer are in contact with each other in area is equal to or larger than10% and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.
 19. Thesensor element according to claim 8, wherein a portion in which theouter leading-end protective layer and the inner leading-end protectivelayer are in contact with each other in area is equal to or larger than10% and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.
 20. Thesensor element according to claim 9, wherein a portion in which theouter leading-end protective layer and the inner leading-end protectivelayer are in contact with each other in area is equal to or larger than10% and equal to or smaller than 50% of a range in which the outerleading-end protective layer surrounds the element base in area.